The present invention relates to a safety switching device for switching on or off switching a technical installation in a failsafe manner, and more particularly to a safety switching device having an improved internal power supply unit.
A safety switching device and a failsafe control/evaluation unit, respectively, in terms of the present invention is a switching device and a control/evaluation unit, respectively, that comply at least with PI d (performance level) in accordance with Standard EN ISO 13849-1 and/or SIL 2 (safety integrity level) in accordance with EN/IEC 62061. In particular, this includes switching devices, safety controllers and also sensor modules and actuator modules that are used for controlling and implementing safety-critical tasks in the field of industrial production environment. In particular, switching devices are known that monitor the operating position of an emergency-off button or of a protective door or by way of example the functional state of a light barrier, and in response thereto switch off a machine or machine area. A failure of safety switching devices of this type can have life-threatening consequences for the operating personnel and for this reason safety switching devices may only be used if they are approved by the relevant regulatory authorities.
In general, a safety switching device of this type comprises an input for connecting a signaling device, an output-sided switching device for failsafely switching off the load, a control/evaluation unit that controls the switching device in response to the signaling device, and a power supply unit for providing an operating voltage.
The power supply units used in safety switching devices are generally used to provide specific operating voltages for the electrical components of the safety switching devices. In general, a higher D.C. voltage or A.C. voltage is converted into one or multiple smaller operating voltages.
In the case of safety switching devices, an external power supply unit is usually used as protection against hazardous body currents in order to limit the maximal output voltage even in the case of defective components to a value that is not hazardous for humans. In the case of common safety switching devices, this maximal output voltage that must not be exceeded is usually 65 V, wherein the nominal voltage for the operation of the safety switching devices is generally 24 V.
In order to supply energy to the electrical components, the safety switching device comprises a further power supply unit that is usually integrated in the safety switching device and converts the nominal voltage of 24 V, by way of example, which is provided by the external power supply unit, into the required lower operating voltages, such as 3.3 V, 5 V or 12 V by way of example. These power supply units must be embodied so as to manage overvoltages of up to 65 V, so that even if there are any defective components within the power supply unit an overvoltage does not damage the components of the safety switching device that are to be supplied with energy.
An overvoltage protection device of this type for a safety switching device is known by way of example from DE 10 2008 051 514 A1. The known device comprises a comparator and an electrically controllable current switch-off means that interrupts the current path to the safety module in the event of an overvoltage. A disadvantage in the case of this overvoltage protection is that a defective component in the overvoltage protection circuit, in particular in the current switch-off element, can lead to an overvoltage even in the components that are to be supplied with energy and said overvoltage can damage said components that are to be supplied with energy.
DE 44 36 858 C2 discloses a further overvoltage protection device having multiple limiter elements that are connected in series, wherein this series connection is arranged between two current connections that are to be limited. The sum of forward voltages of the limiter elements defines the maximum permissible voltage between the current connections. By means of partially bridging the individual limiter elements and by reading back the voltage change at the limiter elements, it is possible to test the operativeness of the limiting elements and thus of the entire protection device. A disadvantage of this overvoltage protection device is that this measure is only effective against transient overvoltages. However, an overvoltage that is constantly present can cause damage to the components of the overvoltage protection device and thus result in an unsafe state.
Overall, a further overvoltage protection device for power supply units is known as a clamping circuit. The output voltage is monitored in the case of a clamping circuit and in the event of an overvoltage the input is short-circuited to ground by means of a thyristor. This leads to a fuse, by way of example a protective fuse, triggering and the current supply being permanently interrupted. In contrast to the previously mentioned overvoltage protection devices, the clamping circuit has the advantage that the current supply is safely and permanently interrupted. However, a disadvantage of a clamping circuit resides in the fact that the operativeness of the circuit can only be tested if the input of the power supply unit is actually short-circuited to ground and the fuse triggers. It is consequently not possible to perform a test during the running operation.